If only a few microbes enter the body, they will probably be overcome by the host's defenses. However, if a large number of microbes gain entry, the stage is probably set for disease. The likelihood of disease increases as the number of pathogens increases. The virulence of a microbe is often expressed as the ID50 (Infection Dose needed to infect 50% of a sample population). A highly virulent microbe could cause a disease with a smaller number of cells.